The sympathetic nervous system is integral to blood pressure regulation through changes in peripheral vascular resistance. Sympathetic nerves typically innervate major feed arteries and extend through the precapillary arteriolar network, but not into capillaries, venules or collecting veins. During development, axon extension closely follows arteries as neurons approach and innervate target organs. This close proximity to blood vessels suggests that vessel derived molecules may play a role in axon outgrowth and target innervation. Due to the selective sympathetic innervation of pre-capillary vessels, our hypothesis is that molecular cues that could promote chemoattraction between arteries and neurons or chemorepulsion between veins and neurons exist and may be expressed by vascular cells. Our specific aims are designed to allow us to identify candidate molecules that may drive selective vascular sympathetic innervation and evaluate their functions in vitro. We will (1) characterize sympathetic innervation patterns and synapse formation by immunofluorescence in several blood vessels in late embryonic and postnatal animals, (2) conduct a subtractive hybridization of innervated and non-innervated vessels to generate a list of differentially expressed candidate molecules and validate their expression through western blot and immunofluorescence and (3) evaluate the functional impacts of any identified candidate molecules using a three-dimensional (3-D) co-culture model using superior cervical ganglion explants with innervated and non-innervated vessel segments to assess directed neurite outgrowth in response to the application of identified candidate molecules to the culture medium. Relevance: Alterations in vascular sympathetic has been implicated in diseases such as hypertension (hyperinnervation) and the pathological sequalae following cardiac transplant (denervation). For example, loss of sympathetic neurons after transplant compromises the ability to regulate peripheral blood flow and to adequately tolerate exercise, temperature fluctuations and other physical stresses. By understanding the molecular mechanisms governing vascular sympathetic innervation, therapies may be developed that would allow for the repair of peripheral vascular sympathetic nerve patterning and therefore allow patients to regain normal function.